A Moineau type positive displacement device can be used as a motor or pump by designing the rotor and stator for the device with a particular shape such as a spiral-helix screw shape to provide a progressive cavity between the rotor and the stator. When operated as a pump, the rotor turns within the stator casing fluid to be moved along the progressive cavity from one end of the pump to the other. When operated as a motor, fluid is pumped into the progressive cavity of the device so that the force of the fluid movement causes the shaft to rotate within the stator. The rotational force can then be transmitted through a connecting rod and drive shaft. Thus the positive displacement device using a specifically designed rotor and stator can be used as a motor or pump depending whether the force of the fluid is pumped through the motor whereupon it functions as a motor or external force acts on the rotor and causes the fluid to move so that it functions as a pump.
In the most basic form of drilling oil and gas wells, a rig motor supplies power to the many lengths of pipe comprising the drill string, causing it to rotate and turn the drilling bit at the bottom of the hole. Turning the drill string from the surface results in a great deal of friction and torsional stress in the upper portion of the drill string. Friction between the drill pipe and the side of the well bore, together with the elastic stretch and twist in the drill pipe, cause an inconsistent weight to bear on the bit. This is harmful to the bit and can also result in metal fatigue failure in the drill string. Therefore, it is often advantageous to utilize a motor at the bottom of the hole as the motive force for the drilling bit, eliminating the need to rotate the drill pipe. This results in reduction of wear on the equipment, lowering of drilling weight requirements, simplification of bottom hole drilling assemblies, and improved cost effectiveness. Directional guidance control is also possible with such systems. Such a motor is less costly to run in many cases. A particular design of motor that is especially well suited to downhole applications is the positive displacement motor discussed above in which a screw-shaped rotor is turned within a stator by a fluid which is pumped through the motor under pressure. The rotational force is then transmitted through a connecting rod and drive shaft to the bit. In motors of this kind, the rotor is generally made of alloy steel bar having a central hole for fluid passage and shaped as a spiral helix and the stator is a length of tubular steel lined with a molded-in-place elastomer. The elastomer is formulated to resist abrasion and deterioration due to hydrocarbons and is shaped as a spiral cavity, similar to but not identical with, the spiral shape of the rotor. In addition to having a basic spiral shape, the rotor may be fluted, with as many as 10 or more flutes. The mating stator will thin have as many flutes, plus one. With proper mutual shaping, the rotor and stator form a continuous seal along their matching contact lines and also form a cavity or cavities that progress through the motor from one end to the other end as the rotor turns. The efficiency of these motors is highly dependent on precise dimensional matching of the rotor and stator profiles.
In operation, drilling fluid or "mud" (usually a mixture of water and/or oil, clay, weighting materials, and some chemicals formulated to fluidize the cuttings made by the drilling bit and to contain formation pressures) is pumped down the length of the motor between the rotor and the stator, causing the rotor to turn and drive the bit. The solids content of the drilling fluid acts to abrade the components of the positive displacement motor, particularly the rotor, while the aqueous environment and chemical substances present often tend to promote corrosion of the rotor. Wear and corrosion of the rotor tend to destroy the designed-in seal between rotor and stator and degrade the performance of the motor to the point that it becomes necessary to remove it from the hole and rework or replace it. Rough, angular, or irregular surface areas that develop on the rotor due to its erosion or corrosion can abrade or cut the mating elastomer, thus degrading the motor operation even when the damage to the rotor is within limits that would be tolerable were it not for the damage to the stator elastomer. While a certain amount of replacement is unavoidable and might have to be done anyhow to change bits to conform to the properties of the various strata through which the hole is drilled, premature wear or corrosion entails, in addition to the cost of reworking or replacing the motor components, the additional expense of pulling the drill string prematurely from the hole. Chrome plate is often applied to the rotor surface to protect it from abrasion and corrosion, but this is not usually satisfactory because it does not have adequate abrasion resistance and because liquid penetration of the chrome plate permits corrosion of the rotor base material. Furthermore, it is difficult to obtain a uniform thickness of chrome plate on the rotor surface because the complex geometry of the rotor causes non-uniform electric fields to develop around the rotor during plating resulting in development of an uneven coating thickness that distorts the designed precise geometrical matching of the rotor with the stator and degrades the efficiency of the motor even when new. In other attempts to protect the rotor from wear and corrosion, nickel-based alloys have been applied to the rotor surfaces by deposition techniques such as plasma spray or other thermal spray device. Coatings of this type may be potentially superior in some ways to chrome plate in erosion and corrosion resistance, but require densification by fusing, hot isostatic pressing, or some other thermal method to seal their inherent porosity so that the rotor substrate is isolated from the corrosive surroundings. Any heat treatment applied to the rotors during the processing of the coating can distort the shape of the rotors with the same resultant mismatch and efficiency losses mentioned above.
It is an object of the present invention to provide a coating for a rotor of a positive displacement motor or pump that has excellent wear ant corrosion resistance characteristics.
It is another object of the present invention to provide a metal carbide and/or metal boride coating for helical shaped rotors for use in positive displacement pumps or motors.
It is another object of the present invention to provide a rotor for a positive displacement motor or pump having an excellent wear-resistance and corrosion-resistance coating.
It is another object of the present invention to provide a cost effective coating for rotors that will extend the useful life of positive displacement devices using such rotors.